You need to include jquery first.
Use this to code to include jquery 
 require(['jquery'], function($){ 
     //your code to send ajax request here
 
  $.ajax({
 showLoader: true, 
url: "admin/PostaPlus/PostaPlusShipping/HSCodes", 
data: param,
 type: "POST", 

 dataType: 'json' 
}).done(function (data) { alert(data); console.log(data); });

Here is my custom controller codes,



});